The build up of insulating films on the elements of an electron beam optical system which become electrically charged by scattered and stray electrons and that then introduce instability in the performance of the electron beam optical system, is quite common in electron microscopes and in electron-beam lithography systems where such contamination occurs for several reasons. The most common reasons are pump contamination, the presence of hydrocarbons as residual gas released from a test specimen and which are polymerized by the electron beam and the release of hydrocarbons from an irradiated organic workpiece, e.g. organic resist materials used on a semiconductor wafer being processed in an electron-beam lithographic system. An additional source of contamination occurs from the formation of metal oxides on the surface of metal lens and deflector elements of the optical system.
Such contamination of the elements of the electron optical system in electron microscopes and electron lithographic systems, and the like presents serious limitations to the use of such instruments. For example, in the past to obviate the effects of such build up of contaminants, elements used in the optical systems had to be designed to be removable for ready cleaning. This complicates the mechanical design as well as resulting in recurring extensive machine down times. In an effort to decrease the rate of contamination and consequent machine down time, some electron beam optical systems have incorporated separate heaters to keep the optical system elements hot and hence inhibit formation of contaminating films. In some instances contamination of lens and deflectors is reduced by using large diameter elements. This latter constraint has resulted in the attempted use of magnetic optical systems instead of electrostatic optical systems in a number of applications. This is due to the fact that large diameter electrostatic lens and deflectors require excessively high voltages for many practical applications.
If small diameter electron beam optical elements could be kept clean readily and economically, then it becomes possible to make minaturized electrostatic electron beam optical columns having a volume many hundreds of times less than the volume of comparable presently used magnetic electron beam columns. One important use of such minaturized electron beam columns is in electron-beam array lithography systems such as that illustrated and described in U.S. Pat. No. 4,390,789 issued June 28, 1983 for an "Electron Beam Array Lithography System Employing Multiple Parallel Array Optics Channels and Method of Operation"--Donald O. Smith and Kenneth J. Harte--inventors and assigned to Control Data Corporation; and U.S. Pat. No. 4,430,571 issued Feb. 7, 1984 for a "Method and Apparatus for Exposing Multi-Level Registered Patterns Interchangeably Between Stations of a Multistation Electron Beam Array Lithography (EBAL) System"--Donald O. Smith and Kenneth J. Harte--inventors and assigned to Control Data Corporation. There is no known way to build miniaturized electron optics systems such as described in these two patents using magnetic lens and deflectors small enough for the applications for which the systems are intended. Furthermore, even if small magnetic columns could be built, interaction due to the fringing magnetic fields between closely spaced columns as described in these patented systems, would make the miniaturized magnetic electron beam columns unsuitable for such uses. However, the necessity for removing contaminated parts from a working electrostatic electron optical system for cleaning makes the use of electron optics in general, and electrostatic optics in particular, impractical for many applications. This also is true of the electron beam lithographic systems described in the above-cited patents. It would be impractical to design such electron beam lithographic systems (EBAL systems) using readily disassembled and removable electron beam optical system elements due to the prohibitive cost and commercially impractical down times that such removal and cleaning would entail.